We were also busy fighting the Japs pretty much on our own, so that Japan had no desire to mess with the Soviets during that time.
Had Japan been freed up to attack the Soviets in Siberia, that would have definitely been a problem for the Soviets.
The Soviets inflicted a terrible defeat on the Japanese Army in Mongolia in 1939 using air/armor combined arms. The Japanese had never seen anything like it and got their butts kicked and never undertook any military action threatening the Soviets again.